1. Field of the Invention
The present invention relates to a method for producing hematite for ironmaking, and more particularly, to a method for producing hematite for ironmaking in which a process of hydrometallurgical refining of a nickel oxide ore includes a plurality of solid-liquid separation processes, thereby making it possible to suppress mixing of a sulfur compound into a leach residue during an real operation.
2. Description of the Related Art
Nickel has been widely used as a raw material of stainless steel.
However, in accordance with the tendency of depletion of a sulfide ore that becomes a raw material of nickel, a technology of refining a low-grade oxide ore has been developed and has been into practical use.
Specifically, a producing process called “high pressure acid leach (HPAL)” has been put into practical use, in which process a nickel oxide ore such as limonite and saprolite is put into a pressurizing apparatus such as an autoclave in combination with a sulfuric acid solution, and nickel is leached under a high temperature of approximately 240° C. to 260° C. and a high pressure.
The nickel leached into a solution of the sulfuric acid is used as nickel metal or a nickel salt compound by adding a neutralizing agent to the nickel so as to neutralize a surplus acid, separating from a leach residue by solid-liquid separation, separating impurities to recover the leach residue as an intermediate raw material in the form of hydroxide or sulfide, and further refining the intermediate raw material.
In the preliminary neutralization process of neutralizing the surplus acid, adjustment of pH that is appropriate for the solid-liquid separation is performed, and then in a subsequent process, or the solid-liquid separation process, concentration of a solid content and solid-liquid separation are performed with a facility called Counter current decantation (CCD). Typically, in the CCD, a plurality of continuous stages of thickeners are used.
A liquid component (hereinafter, may be referred to as an overflow), which is obtained from the CCD, is returned to a neutralization process for adjustment of pH that is appropriate for a sulfurization process. The pH adjustment is performed to remove a fine solid content that occurs through precipitation. Then, the liquid component is transmitted to the sulfurization process, and the liquid component is subjected to a sulfurization treatment, thereby an intermediate raw material such as a mixed sulfide of nickel and cobalt is typically obtained.
In this regard, for example, JP 2004-225120 A discloses a technology in which a part of a solid content (hereinafter, may be referred to as an underflow), which is obtained in CCD, is added as a seed crystal for a neutralization process so as to promote generation of a fine precipitate. Actually, this technology has been effectively used for an improvement in real-operation efficiency.
Employing the producing process called high pressure acid leach (HPAL) makes it possible to leach nickel almost completely, for example, in the case of nickel oxide ore, even in a low-grade ore in which a target valuable metal to be recovered is contained in an amount of 1% by weight to 2% by weight (hereinafter, the grade will be expressed by “%”).
In addition, the intermediate raw material is manufactured from a leachate, and thus a target metal is concentrated to the same extent as in a conventional raw material, and the target metal can be obtained through substantially the same refining method and refining process as in the conventional raw material.
Further, the HPAL process is applicable to not only the nickel oxide ore but also many kinds of ores such as a nickel sulfide ore, a copper sulfide ore, and a copper oxide ore.
Besides, a main component of the leach residue that is obtained by the HPAL process is an iron oxide in a type of hematite and the like, and approximately 50% of iron is contained in the leach residue. Production volume of the leach residue is approximately 50 times to 100 times as much as that of the intermediate raw material. The reason for this is that each of the nickel oxide ore or the copper sulfide ore of a raw material contains iron in an amount much more than that of nickel or copper.
The leach residue is generated at a high temperature, and is in a type of a chemically and environmentally stable oxide, but has no particular utility value in a current state, and has been thus scrapped and stored in a residue disposal yard.
Therefore, a broad residue disposal yard is necessary for scrap and storage of an enormous amount of the leach reside which is generated in accordance with the HPAL process operation.
In steel smelting, a method of charging iron ore containing iron oxide into a blast furnace along with a reductant such as coke, heating and melting the iron ore under a reducing atmosphere to obtain crude steel, and refining the crude steel in a converter to obtain desired steel has been used.
The iron oxide that is a raw material of the steel is a limited resource, and furthermore it is gradually hard to obtain high-quality iron ore required to maintain a quality of steel. Accordingly, a study has been made with respect to use of the leach residue as the iron ore.
However, the leach residue in the HPAL process cannot be directly used as a raw material for ironmaking. The reason is that the leach residue in the HPAL process contains vein stone or impurities, particularly sulfur, in addition to the iron oxide, and thus the leach residue is not appropriate for a raw material that is used in a conventional iron-making process in common. Specifically, this is because the sulfur grade is high.
Particularly, a grade of the sulfur in the iron oxide which can be used as a raw material for ironmaking is different depending on facility capacity, an amount of production, and the like in individual ironworks. Typically, it is necessary to suppress the sulfur content to less than 1%.
Typically, the leach residue contains approximately 5% to 8% of sulfur.
The majority of sulfur contained in the leach residue is derived from calcium sulfate (plaster) that is mixed in during nickel refining.
When neutralizing free sulfuric acid, which remains in a leach slurry obtained during high-pressure acid leaching (the free sulfuric acid is sulfuric acid that remains without reaction in the sulfuric acid that is excessively added for performing sufficient leaching in the HPAL process), a typical inexpensive calcium-based neutralizing agent, for example, limestone or slaked lime is added. Accordingly, when calcium contained in the neutralizing agent and the free sulfuric acid react with each other, the plaster is generated and is then mixed into the leach residue.
A part (approximately 1%) of sulfur that is contained in the leach residue is trapped inside particles of hematite produced.
Thus, it is assumed that it is preferable to use what forms a soluble salt as a neutralizing agent to be added, instead of what forms insoluble precipitate, such as limestone or slaked lime, after the neutralization.
Examples of the neutralizing agent that is appropriate for the use include sodium hydroxide, potassium hydroxide, magnesium hydroxide, magnesium oxide.
However, From reasons including that these neutralizing agents are expensive and a production amount thereof is small, these neutralizing agents are not appropriate for a process such as the HPAL process in which an enormous amount of neutralizing agent is consumed.
Accordingly, it has been inevitable to totally or partially use the calcium-based neutralizing agent that forms the insoluble sediment after neutralization, and it has been impossible to avoid mixing-in of sulfur. Accordingly, it has been difficult to use hematite obtained by processing the leach residue produced in the HPAL process as the raw material for ironmaking.
Meanwhile, there is also known a method of separating sulfur in jarosite by using a pressurizing apparatus such as an autoclave.
For example, JP 03-176081 A discloses a method that includes stirring a jarosite-containing residual and a zinc sulfide inclusion in an autoclave at least under oxygen partial pressure of 1000 kPa at a temperature of 130 to 170° C. along with a free sulfuric acid of 40 to 100 g/l, substantially dissolving iron and zinc fractions of a concentrate containing the residual and zinc sulfide, introducing the solution into a leach circulation passage for zinc electrolysis to settle iron in the form of hematite, and separating sulfur from the above solid, and supplying the residual for separate application.
However, this method has problems of requiring an expensive device such as an autoclave, increasing a facility cost, and having a problem even in the aspect of productivity.
Then, it has been considered to use magnesium oxide contained in the ore as the neutralizing agent.
For example, JP 2009-520661 A discloses a process of recovering magnesium oxide from a source of magnesium sulfate. The process includes the steps of: preparing a source of magnesium sulfate in a solution that is derived from part of a process associated with leaching of a metal-containing ore or concentrate; converting the magnesium sulfate in solution into solid magnesium sulfate; contacting the solid magnesium sulfate with elemental sulfur in a reducing atmosphere; and recovering the magnesium as magnesium oxide, and the sulfur as a sulfur dioxide gas.
Using this method makes it possible to reuse magnesium contained in the ore as a neutralizing agent, and to suppress calcium that is carried, thereby reducing calcium that is mixed into iron oxide in the residue.
However, in the method disclosed in JP 2009-520661 A, a large amount of heat is necessary to crystallize magnesium in the solution as magnesium sulfate, or to heat the obtained magnesium sulfate for conversion into a magnesium oxide, and thus it cannot be said that the method is economical.
In this regard, there has been suggested a method of using an oxide ore (limonite ore), in which the magnesium content is high, as the neutralizing agent.
For example, JP 4294685 B1 discloses a method of recovering nickel or cobalt from an oxide ore containing nickel or cobalt, and iron. The method includes the steps of: preparing a first oxide ore and a second oxide ore as the oxide ore, the second oxide having higher magnesium content than the first oxide ore; classifying the first oxide ore into a first small-particle-size oxide ore and a first large-particle-size oxide ore, and classifying the second oxide ore into a second small-particle-size oxide ore and a second large-particle-size oxide ore; leaching nickel or cobalt from the first large-particle-size oxide ore with sulfuric acid to obtain a sulfuric acid leachate containing nickel or cobalt, and a leach residue; mixing the sulfuric acid leachate containing the leach residue with the second large-particle-size oxide ore to react the sulfuric acid leachate with magnesium contained in the second large-particle size oxide ore for adjusting a pH, thereby obtaining a reaction solution containing nickel or cobalt, and a reaction residue containing iron; and neutralizing the reaction solution containing the reaction residue with a neutralizing agent to obtain a neutralization solution containing nickel or cobalt, and a neutralization residue containing iron.
When using this method, it is possible to use the nickel oxide ore as the neutralizing agent.
However, the cost and time for classification of the ore are significant. In addition, a large amount of vein stone components are contained in the leach residue, and thus the iron content is low. Accordingly, it cannot be said that the leach residue is an efficient raw material.
Accordingly, it has been difficult to substitute the total of the neutralizing agent that is used in the HPAL process with magnesium oxide.
In addition, a method of preventing sulfur from being mixed-in by substituting the neutralizing agent with magnesium oxide derived from a base rock only in the preliminary neutralization process of producing the leach residue is easily retrieved.
However, when using the technology of improving real-operation efficiency as is described in JP 2004-225120 A, with the conventional calcium-based neutralizing agent in the neutralization process, the residue in the neutralization process is returned to CCD, and thus it is difficult to avoid mixing of sulfur into the leach residue.
An object of the present invention is to provide a method for producing hematite for ironmaking which is capable of using a base rock-derived neutralizing agent other than a Ca-based neutralizing agent and a conventional Ca-based neutralizing agent during a real operation of refining hematite, which has such a low sulfur component as to be used as a raw material for ironmaking, from a leach residue containing iron oxide that is produced by an HPAL process.